1. Technical Field
This disclosure relates to current sensing circuits, including current sensing circuits that control motors driven by an H-bridge of electronic switches.
2. Description of Related Art
The windings of a motor may each be driven by an H-bridge of electronic switches, which may be driven by a pulse-width-modulated (PWM) signal.
The system may need to continually measure the current flowing into (or out of) each motor winding. To effectuate this needed measurement, a shunt resistance may be placed in series with each winding, and the differential voltage across each shunt resistance may be measured and serve as an indication of the current that is traveling through it.
A differential amplifier may be used to measure the differential voltage across the shunt resistance. However, the differential voltage across the shunt resistance may be very low, while the common mode voltage of the shunt resistance—the instantaneous average of the voltages of the shunt resistance terminals with respect to ground—may vary widely as the H-bridge is switched by the PWM signal. This may cause errors in the measurements made by the differential amplifier.
To reduce these errors, the differential amplifier may have a high common mode voltage rejection ratio (CMRR). This may enable the differential amplifier to accurately track the differential voltage across the shunt resistance, even in the face of wide variations in the common mode voltage. However, difficult trade-offs between optimizing for DC accuracy (offset voltage) and AC performance (AC CMRR) may be needed.
Some H-bridge driven motor controllers may not directly measure the current in the motor winding. Instead, they may measure the current in either a ground or a supply connection to switches that form the H-bridge. However, the information provided by this approach may not be as accurate as directly measuring the current to the motor winding.